The greenhouse effect due to CO2 has been pointed out as one of the causes of global warming, and there is an urgent need to take global measures against the greenhouse effect to protect the global environment. The sources of CO2 can be found in various fields of human activities in which fossil fuels are burnt, and there is an increasing tendency to tighten CO2 emission regulations. Accordingly, extensive studies have been conducted on CO2 recovery methods applicable to power generation facilities, such as thermal power plants, which use a large amount of fossil fuels. In these methods, flue gas from a boiler is brought into contact with an amine-based absorbent such as an aqueous solution of an amine compound to collect and recover CO2 in the flue gas.
When such an absorbent is used to recover CO2 from flue gas, the amine compound is entrained in the decarbonated flue gas in which the amount of CO2 has been reduced. Therefore, to prevent air pollution by the amine compound, the amount of the amine compound emitted together with the decarbonated flue gas must be reduced.
PTL 1 discloses a conventional amine recovery system. This system includes a plurality of stages of water-washing units for collecting an amine compound entrained in decarbonated flue gas. In each water-washing unit stage, wash water is brought into gas-liquid contact with the decarbonated flue gas in which the amount of CO2 has been reduced by gas-liquid contact with an absorbent that absorbs CO2. The amine entrained in the decarbonated flue gas is collected sequentially in the plurality of stages of the water-washing units. As the wash water used in PTL 1, condensed water is used which is produced by condensing and separating water contained in CO2 during a process of regenerating the amine-based absorbent by removing CO2 from the CO2-absorbed amine-based absorbent.
PTL 2 discloses a conventional decarbonation system including: a cooling unit for cooling decarbonated flue gas in which the amount of CO2 has been reduced by gas-liquid contact with an absorbent; and a contact unit in which condensed water condensed in the cooling unit is brought into countercurrent contact with the decarbonated flue gas. PTL 2 discloses another decarbonation system that includes a water-washing unit for collecting the amine compound entrained in decarbonated flue gas by bringing wash water into gas-liquid contact with the decarbonated flue gas in which the amount of CO2 has been reduced by gas-liquid contact with an absorbent. The wash water used is condensed-water condensed in a cooling tower for cooling the flue gas from which CO2 has not been recovered.
In a conventional method disclosed in PTL 3, sulfuric acid is sprayed into decarbonated flue gas in which the amount of CO2 has been reduced by gas-liquid contact with an absorbent. The amine compound entrained in the decarbonated flue gas is thereby converted to a basic amine compound sulfate. Then the decarbonated flue gas containing the sulfate of the basic amine compound passes through a demister to collect the sulfate of the basic amine compound from the decarbonated flue gas.